MACROPORE CHARACTERIZATION BY INDIRECT METHODS

Macropores are important in preferential flow; therefore, function and continuity of macropores are often more important than visual observation. The objective of this research was to compare several indirect methods of determining macropore volume. Ponded and tension infiltration measurements were made in situ at the soil surface and 0.25 m deep in a Waukegan silt loam (fine-silty over sandy, mixed, mesic Typic Hapludoll), and air-filled porosity at each pressure head was calculated from conductivity determinations. Undisturbed, unconfined samples were taken from the 0.25- to 0.35-m depth. Air-filled porosity as a function of pressure head was determined on these samples using wetting and draining equilibrium curves in rotated cores (to even out the gravitational gradient), measured/calculated after steady-state lab conductivity, and calculated from ped size distribution (assuming a ped shape factor and ped size/crack size relation, with known bulk density and ped density). Water adsorption/desorption in rotated cores in only four out of 20 samples displayed an ''air-entry'' pressure head, and no samples displayed a ''water-entry'' pressure head (that is the head at which no more water enters pores during equilibrium soil water adsorption). All methods tested estimated air-filled porosity in the same range. Air-filled porosity determined from ped-size distribution and from ponded and tension infiltration measurements was much less tedious than from rotated core measurements.